Truro Anticlinal structure

TP-EB rtt r-l

CHAPTER 7. CHAPTER 7. MACROSCOPrc STRUCTURES 95

7.3.2 Truro Anticlinal structure

The anticlinal

structures

in the Truro-Dutton

region have

not

been

formerly

defined and are therefore referred

to

here ^as the Truro

Anticlinal

structure. The Truro

Anticlinal

structure has been mapped by Coats and Thomson (1959) and

in

part by Kleeman (1965) and Fleming (1965).

In

Figurã

1.2,the

major anticlinal structure immediately

to

the west of the Karinya Syncline is

the Truro Anticlinal structure.

^The

Truro Anticlinal

structure is made

up

of two major folds.

The northern fold is a southerly plunging anticline which is defrned ^as the north Truro Anticlinal structure

(NTA).

South of this there lies a doubly plunging anticline and this is the south Truro

Anticlinal

^structure

(STA).

The axis of the

NTA

and of the STA

both

trend approximately ^NS.

Mancktelow (1979) interpreted the Karinya Syncline and the

NTA

to be

Ir, ^but

^{the STA to}

be

^F2. Since Adelaide Supergroup rocks on the west limb of the Karinya Syncline form the east

limb of the NTA, ^the NTA

^and

^the

^Karinya Syncline are

likely to

be

of the

same generation.

However

the

south plunging STA and

the

south plunging

Karinya

Syncline cannot be sirnply connected ^(see above). The

NTA

and the

sTA

are

both

F*o¿n folds.

NTA: north Truro Anticlinal ^structure

The regional structure consists of ^a north-plunging anticline

with

Adelaide Supergroup ^meta'sed- iments

in the

core of

the

anticüne (Figure 7.16).

Minor faulting prior to F^oin

resulted

in

the d.isconnected strata on the limbs of the

fold.

IVlagnetic anomalies

in

the Tindelpina Shale mem- ber of the Tapley

Hill

Formation and

in

the Tarcowie Siltstone delineate the

fold.

The magnetic anomaly US-T has been identified as the Ulupa Siltstone magnetic marker (Section 6.1.3). Tlie east limb of the

NTA

continues ^as the west limb of the

l{arinya

Syncline which is consistent rvith

both

folds belonging

to the

same generation. The

fold

plane

is tilted to the

west and ^plunges 55o

in

a direction 195o (Kleeman, 1965).

An

angular unconformity between the Heatherdale Shale (Normanville Group) and the un- derlying

Àd"lrid"

Supergroup was inferred

by

Kleeman (1965)

but the

uppermost

unit of

the Adelaide Supergroup mapped

by him is

below

the

Ulupa

Siltstone. The

Heatherdale Shale ^is significant ^because volcanics are intercalated

with it.

Following US-T from the Australia Plains aÃu,

*her"

its identification is reliable, to the eastern limb of the NTA,

it

^was surprising to note

that

the Ulupa Siltstone has

not

been mapped

in

the region'

In

the region of the type

locality

of the Truro Volcanics, work done

by

Forbes et al. (1972) and Kleema" IfOOS¡ shows Adelaide Supergroup rocks (below the Ulupa Siltstone) ^as cutting out against the Normanville Group. The volcanics have been identified as Cambrian and described

,s inter.alated with the

Heatherdale Shale. As Figure 7.13 shows,

the

source of

the

magnetic anomaly, US-T, is

right

below

the

mapped occurrence of the volcanics. The main alternatives

to

consider ^are:

1. Anomaly US-T

has been mistakenly

identified

as being caused

by the Ulupa

Siltstone when

it

^is

in fact

caused

by

Heatherdale Shale. The Heatheldale Shale

in

the region ^has

CHAPTER 7.

MACROSCOPrc STRUCTUNES ⁹⁷

abundant, fine-grained ^opaques which could ^be magnetite. In the

wMZ

and CMZ, after the anomalies caused

by

the Barossa Complex rocks,

the

most distinctive magnetic anomaly is caused by the Ulupa Siltstone. Wherever the Ulupa Siltstone rocks have been mapped, there is a magnetic anomaly associated

with it

(Figure 7.13). The magnetic anomalies ^are of

the

order of several hundreds of nanoTeslas, and even show up on

the

SADME ⁽¹⁹⁸⁰⁾ maps. Samples of the Ulupa Siltstone have been taken from areas where the identification

is

r-eliable

(belamere: Brotherton

(1967), Mancktelow (1979);

Mt.

Barker Creek: Toteff (1922);

Australia

Plains: Drummond ^(1972), Mancktelow (1979)) and the high magnetic susceptibilities, strong

Q

^lactot, and percentage and constituents of the magnetic mineral assemblages, are

ull in

fol oor

of the

Ulupa Siltstone being

the

main magnetic marker in

the CMZ

and

WMZ.

Anomaly US-T was followed

from the Australia

^{Plains area where}

it

coincides

with

outcrop of the Ulupa Siltstone'

2.

The geologic mapping

in the

area is incorrect and

the

rocks ascribed

to the

Heatherclale Shale really belong

to

ttre Ulupa Siltstone. The sedimentology of the rocks

in the

area is

very similar to thát of the

Heatherdale Shale (Gatehouse, pers'

comm.)

and unlike the

grulo-gr"y

siltstones which are characteristic of the Ulupa Siltstone. ^So

it

is unlikely

that

ih"

^Trrrro Volcanics are

in

^the Precambrian and

not in the

Cambrian. The

identificatiol

could

be

tested

by

measuring

the titanium

^content

of the

so-called Heatherdale ^Shale.

The high

titanium

content of the Ulupa Siltstone ^(see Table 2.7) could perhaps be used to ,ilistinguish

it ^from

siltstones of the Heatherdale Shale'

3.

The contact between

the

Cambrian and the Precambrian

in this

region, which has never clearly been explained before,

is not

simply an

ulconformity

folded

by the F^oin

^defor'- mation event.

And

therefore ^as the cross section shown

in

Figure 7.16 implies, the Ulupa Siltstone is near-surface ^as shown, though

it

is overlain by the Heatherda'le Shale.

All

the dips are steeP'

As explained above,

I

prefer the third, alternative, i.e. US-T is ^caused by the Ulupa Siltstone;

the

Truro

Volcanics are intercalated

with

the Heatherdale Shale; and though the dips are ^steep,

the

Heatherdale Shale

is

underlain

by the Ulupa

Siltstone.

It is not

enough

that

^{there be an} unconformity between

the

Precambrian and

the

Cambrian,

the

contact must be

fault

related, and perhaps

part

of the F"orly event.

STA: south Truro Anticlinal ^structure

The STA is a doubly-plunging anticline south of the

NTA.

As in the NTA, Adelaide Supergroup rocks are found

in the

core

of

the

anticline.

^{Fleming (1965)} interpreted the

major

anticline to be a

first

deformation

event.

He found

that the

axial plane dips moderately east and the fold plunge is towards the south and varies

from

steep

to

moderately ^steep.

The

stratigraphy

in

the area is uncertain and different interpretations have been presented

in

the geological maps of Truro (Coats and Thomson, 1959) and of Adelaide (Thomson, 1969b)' Extensive areas

of

outcrop

of the

Angaston

Marble

(Normanville

Group)

are shown on botir maps. The region is ^also known for its phosphate deposits in the Koonunga Phosphorite Member of the Normanville Group (Brown, 1908). The main lithologies ^are marbles (Normanville Group) and scapolite schists and the outcrop is poor.

The magnetic marker, NG3, is,

for

most of

its

length a single magnetic

unit.

Similar linear magnetic anomalies ^elsewhere

in

the study area have all been

stratiform

and there is no ^rea'son

to

believe otherwise

of NG3. It is

possible though

that NG3

may

really

be

in the

Adelaide Supergroup as there

is

confusion

about the

stratigraphy as mentioned

above.

However,

it

^is

t¡ra¡xttooYkxt¡{

ldrsDGFtU¡t -q

I

Figure 7'18: Subdivision of the ISZ

into

subareas. The grey-scale image of

total

magnetic ficld rvas reduced from Plate 3.

CHAPTER 7.

MACROSCOPrc STRUCTURES ⁹⁸

unlikely,

considering

the lower

amplitude

of the

anomaly,

that NG3

could

be in the

Ulupa Siltstone; and though Normanville Group outcrop ^is limited, there are magnetic anomalies

within the

Group (e.g. NG1 and NG2) and the Heatherdale Shale contains fine-grained magnetite.

Mancktelow (1979) decided

that the

STA, unlike

the NTA,

was an Fz

fold'

The structural relationship between

the

STA and the Karinya Syncline cannot be simple. The western

limb

of the Karinya Syncline ^does not form the eastern limb of the STA and the relationship must in part be

a fault contact.

^Fleming (1965) mapped

part of the

STA and concluded

that the

contact between

the

Precambrian

and

Cambrian could

not be stratigraphic but must be

structural.

He inferred a

fault

contact

but

lack

of

outcrop over

the

boundary zone prevented any further confirmation. From

the

available magnetic and geological maps, the following can be inferred:

1.

The stratigraphy on the western and eastern limbs are different. The marker NG3 is seen

on the eastern

limb

of STA

but

has

not

been found on the western limb.

2.

The phosphate deposits

in

the Koonunga Phosphorite Member are underlain by magnetic

Ulupa Siltstone. ^This is

based

on the

presence

of

strong magnetic anomalies over the phosphate deposits.

3.

Though the regional structure ^closes in the south, individual units do

not.

In the southrvest, Ulupa Siltstone has been ^maPPed.

4.

The western

limb

of the Karinya Syncline ^does

not

form the eastern

limb

of the STA.

There are several feasible solutions

but

lack

of structural

mapping

limits the

geophysical

interpretation.

One solution is

that

the effects

of

an earlier deformation have been overplinted by the F^oin STA and Karinya Syncline folds. Alternatively, the STA may be separated frorn the Karinya Syncline

by

a

fault. Tight

"rootless" folds have been observed

in

the area between the STA and the Karinya Syncline.

It

is also possible

that

the Karinya Syncline and STA represent different generations of

folding.

The folding

in this

area is much more complex than previously mapped and

further

structural investigation is required

to

resolve the deformation style.